Process for producing optically active cysteine derivatives

ABSTRACT

A process for producing optically active cysteine derivatives with high optical purity and good quality which is economically advantageous and is high in productivity even on a commercial scale is provided.  
     A process for producing an optically active cysteine derivative which comprises synthesizing a D-form or L-form optically active cysteine derivative of the general formula (2) shown below (R 1  represents an amino-protecting group of the urethane or acyl type, R 0  represents a hydrogen atom or, taken together with R 1 , an amino-protecting group, R 2  represents an alkyl, aryl or aralkyl group, R 3  represents a univalent organic group and * represents the position of an asymmetric carbon) by reacting the corresponding D-form or L-form optically active amino acid derivative of the general formula (1) shown below with an alcohol of the general formula (3) shown below and a strong acid and/or a thionyl halide and recovering the above cysteine derivative (2) from the reaction mixture, the procedural series from reaction to recovery being carried out under conditions such that the medium contacting the above optically active cysteine derivative (2) is within the range from acidic to weakly basic to thereby recover the above cysteine derivative (2) from the reaction mixture while suppressing the decomposition and racemization thereof.

TECHNICAL FIELD

[0001] The present invention relates to a process for producing a D-formor L-form optically active cysteine derivative represented by thegeneral formula (2) (hereinafter, such derivatives are also referred toas “cysteine derivatives (2)”):

[0002] wherein R¹ represents an amino-protecting group of the urethaneor acyl type, R⁰ represents a hydrogen atom or, taken together with theabove R¹, an amino-protecting group, R² represents a univalent organicgroup selected from the group consisting of a substituted orunsubstituted alkyl group containing 1 to 7 carbon atoms, a substitutedor unsubstituted aryl group containing 6 to 10 carbon atoms and asubstituted or unsubstituted aralkyl group containing 7 to 10 carbonatoms, R³ represents a univalent organic group capable of functioning asan ester-type carboxyl-protecting group by its being included in thestructure represented by —COOR³and* represents the position of anasymmetric carbon atom.

BACKGROUND ART

[0003] The cysteine derivatives (2), particularly in L form, which areobtainable by the present invention are compounds of importance asstarting materials for the production of intermediates of HIV proteaseinhibitors. For example, said derivatives are useful as startingmaterials in the reaction scheme shown below, as described in WO96/23756 and EP 604185 Al, for instance.

[0004] A production technology for the above cysteine derivatives (2)which is so far known in the art comprises introducing an R²S group (R²being as defined above) into a corresponding compound whose amino andcarboxyl groups are protected.

[0005] Referring to this technology, a method is known which comprisesconverting the hydroxyl group of a serine derivative to a leaving groupand then carrying out the substitution reaction [Tetrahedron Lett., vol.28, p. 6069 (1987); ibid., vol. 34, p. 6607 (1993); EP 604185 A1].

[0006] In this way, the hydroxyl group of a serine derivative isconverted to a sulfonyloxy group, followed by substitution reaction witha thiol derivative in an aprotic solvent. However, as a result ofinvestigations made by the present inventors, it was revealed that theyield of the desired cysteine derivative (2) is not always high and thatthere are other problems; the quality is poor and, in particular, theoptical purity decreases.

[0007] Further, there is no detailed description of the method ofisolation of the desired cysteine derivative (2), and the above methodcannot be said to be a production technology leading to good yields.

[0008] Thus, any method of producing those cysteine derivatives (2),which are important as raw materials for the production of intermediatesof HIV protease inhibitors, has not been established as yet.

[0009] In view of the current state of the art as mentioned above, theprimary object of the present invention is to provide a process forproducing cysteine derivatives (2) which is economically advantageousand insures high productivity even on a commercial scale, together withhigh optical purity and good quality.

DISCLOSURE OF INVENTION

[0010] Investigations made by the present inventors revealed that theabove cysteine derivatives (2) are unexpectedly unstable and, inparticular, highly basic conditions bring about such undesirable eventsas racemization and decreases in yield. This is presumably becauseabstraction of the hydrogen atom at position a to the carbonyl group orE2 elimination is caused by high basicity, leading to formation of adehydroalanine derivative as a byproduct, and, on the other hand, theelimination product thiol derivative reacts with the byproductdehydroalanine derivative in the manner of Michael addition, as shown bythe following reaction formula:

[0011] It is therefore believed that it is not easy to suppressdecomposition or racemization of the above cysteine derivatives (2) whensuch a prior art technology as the one mentioned above, that generallyrequires highly basic reaction conditions, is used.

[0012] Based on the above finding, the present inventors made intensiveinvestigations in search of a process for allowing the reaction toproceed smoothly and recovering the above cysteine derivatives (2) whilesuppressing the decomposition and racemization of the cysteinederivatives (2) and, as a result, found out a process favorable forcarrying out the procedural series from reaction to recovery underacidic to weakly basic conditions while avoiding highly basicconditions.

[0013] Thus, the present invention is related to a process for producingthe above optically active cysteine derivative (2) which comprisesreacting a D-form or L-form optically active amino acid derivativerepresented by the general formula (1) (hereinafter also referred to as“amino acid derivative (1)”:

[0014] wherein R⁰, R¹, R² and * are as defined above, with an alcoholrepresented by the general formula (3) (hereinafter also referred to as“alcohol (3)”):

R³OH  (3)

[0015] wherein R³ is as defined above, and a strong acid and/or athionyl halide to synthesize a D-form or L-form optically activecysteine derivative represented by the above general formula (2)

[0016] and recovering the above optically active cysteine derivative (2)from the reaction mixture,

[0017] the procedural series from reaction to recovery being carried outunder a condition such that the medium contacting the above opticallyactive cysteine derivative (2) is within the range from acidic to weaklybasic to thereby recover the above optically active cysteine derivative(2) from the reaction mixture while suppressing the decomposition andracemization thereof.

[0018] In the following, the present invention is described in detail.

[0019] In the production process of the present invention, by carryingout the reaction using a strong acid and/or a thionyl halide, thereaction system can be maintained under acidic conditions under whichthe decomposition and racemization of the above cysteine derivative (2)can be completely inhibited. Thus, the reaction system is maintainedunder acidic conditions throughout the process, or the conditionstherein favorably shift to acidic conditions as the reaction proceeds,namely with the formation of the cysteine derivative (2).

[0020] Referring to the above general formula (1) or (2), R¹ representsa urethane-type or acyl-type amino-protecting group. The urethane-typeor acyl-type protective group is not particularly restricted but may beany one having an amino-protecting effect. For example, it can beselected from among those protective groups described in ProtectiveGroups in Organic Synthesis, 2nd edition, published by John Wiley & Sons(1991).

[0021] Under such reaction conditions as mentioned above, urethane-typeand acyl-type protective groups are judiciously used for masking thebasicity of the amino group. Among them, urethane-type protective groupssuch as aralkyloxycarbonyl groups and lower alkyloxycarbonyl groups arepreferred from the viewpoint of ease of handling, inexpensiveness andconvenience in substrate compound synthesis, among others. Inparticular, benzyloxycarbonyl, tert-butoxycarbonyl, methoxycarbonyl andethoxycarbonyl are preferred, and benzyloxycarbonyl is more preferred.

[0022] The above symbol R⁰ usually represents a hydrogen atom. When theabove amino-protecting group is a phthaloyl group or the like, however,it, together with the above R¹, represents an amino-protecting group.

[0023] The above R² represents an alkyl group containing 1 to 7 carbonatoms, an aryl group containing 6 to 10 carbon atoms or an aralkyl groupcontaining 7 to 10 carbon atoms. These groups may optionally besubstituted according to need. Preferred as R² is phenyl, however.

[0024] The amino acid derivatives (1) to which the process of thepresent invention is favorably applicable are those S-phenylcysteines(R²=phenyl) whose amino group is protected by a urethane-type oracyl-type protective group. Among them, those S-phenylcysteines(R²=phenyl) whose amino group is protected by a urethane-type protectivegroup are more preferred, and that S-phenylcysteines (R²=phenyl) whoseamino group is protected by benzyloxycarbonyl, namely D-form or L-formN-benzyloxycarbonyl-S-phenylcysteine (R⁰=hydrogen, R¹=benzyloxycarbonyl,R²=phenyl), is particularly preferred.

[0025] Since, as mentioned above, the R²S group tends to undergoingelimination and, when R²is an aryl group containing 6 to 10 carbonatoms, in particular phenyl, the tendency toward elimination generallyincreases. Even in such cases, the process of the present invention canbe used very favorably.

[0026] Referring to the above general formula (2) or (3), R³ representsa univalent organic group capable of functioning as an ester-typecarboxyl-protecting group through its being included in the structurerepresented by —COOR³, as is evident from the above general formula (2).The above univalent organic group is not particularly restricted but maybe any one having a carboxyl-protecting effect. For example, it can beselected from among those protective groups described in ProtectiveGroups in Organic Synthesis, 2nd edition, published by John Wiley & Sons(1991). Among them, lower alkyl, benzyl, substituted benzyl and likegroups are preferred, lower alkyl groups containing 1 to 4 carbon atomsare more preferred, and methyl is most preferred.

[0027] The term “protecting” as used herein with reference to the aboveR¹ and R³ means that the relevant functional group is in a form modifiedso that no undesirable side reactions can occur.

[0028] The amount of the above alcohol (3) is not particularlyrestricted but the alcohol may be used in an amount not less than about1 equivalent per equivalent of the above amino acid derivative (1).Generally, however, the above reaction is carried out using the abovealcohol (3) in excess relative to the above amino acid derivative (1)Preferably, the reaction is carried out using a large excess of theabove alcohol (3) as a reaction solvent.

[0029] The strong acid to be used according to the invention is notparticularly restricted but includes, among others, inorganic acids suchas sulfuric acid and hydrochloric acid; and organic acids such asp-toluenesulfonic acid, benzenesulfonic acid and methanesulfonic acid.

[0030] Preferred as the above strong acid are those showing a pKa of notmore than 2.0, more preferably not more than 1.0, in aqueous solutions.When the above strong acid is a polybasic acid such as sulfuric acid,the above pKa indicates the value at the first stage of dissociation atwhich that value is minimal. It is to be noted that a compoundsimultaneously having an acidic group and a basic group such as an aminogroup, for example an amino acid, even if it has an acidic group showingsuch a small pKa value, is distinguished from the strong acid to be usedin the practice of the present invention, since it, as the wholemolecule, does not show strong acidity.

[0031] The amount of the above strong acid is not particularlyrestricted but, preferably, the strong acid is used in an amount notsmaller than the catalytic amount, more preferably 0.01 to 0.1equivalent, relative to the above amino acid derivative (1), ifnecessary in a still larger amount.

[0032] When hydrogen chloride is used as the above strong acid, it isgenerally used by blowing into the reaction solvent. For example, it isblown through the above alcohol (3), which also serves as a reactionsolvent, to an extent such that the solvent is saturated with the same.

[0033] The thionyl halide to be used in the present invention is notparticularly restricted but thionyl chloride is preferred from theviewpoint of ease of handling and inexpensiveness, for instance.

[0034] The amount of the above thionyl halide is not particularlyrestricted but, generally, it is used preferably in an amount of notless than about 1 equivalent, more preferably within the range of 1 to 5equivalents, relative to each equivalent of the above amino acidderivative (1). Most preferably, it is used in an amount of about 1equivalent or a slightly larger than 1 equivalent.

[0035] In carrying out the reaction according to the present invention,the three materials, namely the above amino acid derivative (1), theabove alcohol (3) and the above strong acid or thionyl halide aregenerally brought into contact with one another all at once. When,however; the above thionyl halide is used, it is also possible to reactthe above thionyl halide with the above alcohol (3) in advance and thenreact this mixture with the above amino acid derivative (1). Generally,either one of the above strong acid and the above thionyl halide isused. It is also possible, however, to use both the above strong acidand thionyl halide simultaneously.

[0036] The reaction temperature for the above reaction is notrestricted. Generally, however, it is within the range of from thesolidification point of the reaction mixture to about 100° C.,preferably within the range of −20° C. to 80° C.

[0037] The time required for going through the above reaction may varydepending on the species and amounts of the above amino acid derivative(1), alcohol (3) and strong acid or thionyl halide as well as on thereaction temperature. Generally, however, it is 1 to 120 hours. Inparticular, it is preferred that the reaction be caused to proceedalmost quantitatively within 20 hours.

[0038] The reaction solvent to be used in the above reaction is notparticularly restricted. Generally, the above alcohol (3) is used alsoas the reaction solvent. For minimizing the amount of the alcohol (3),an aromatic hydrocarbon, an ether or the like may also be used as thereaction solvent. From the viewpoint, among others, of ease of removal,by azeotropic distillation, of the byproduct water formed in smallamounts, a solvent immiscible with water and facilitating the removal ofwater by distillation is preferred among others, and a solvent capableof forming an azeotrope with water is more preferred. As such solvent,there may be mentioned aromatic hydrocarbons and, in particular,6-membered aromatic hydrocarbons containing 6 to 8 carbon atoms are morepreferred, and a most preferred solvent is toluene.

[0039] The reaction mixture after completion of the reaction effectedaccording to the present invention contains not only the desiredcompound, namely the above cysteine derivative (2), but also the abovestrong acid and/or the hydrogen halide and sulfur dioxide formed asbyproducts and dissolved therein when the above thionyl halide is used;thus, it is in a state containing a very strongly acidic substance orsubstances.

[0040] The above cysteine derivative (2) is favorably protected againstdecomposition or racemization under acidic conditions and, therefore,such strongly acidic substances need not always be removed. If desired,however, they may be eliminated by evaporation or neutralization.Elimination by neutralization is preferred and, when a thionyl halide isused, combined use of removal by evaporation (e.g. degassing underreduced pressure) and elimination by neutralization is judicious.

[0041] In the practice of the present invention, the removal of theabove strong acid and/or the byproduct hydrogen halide and sulfurdioxide is conducted under acidic to weakly basic conditions so that theabove cysteine derivative (2) may be prevented from being decomposed orracemized, as mentioned above. When said conditions are defined in termsof pH, the elimination is carried out generally at a pH not higher than10, preferably at a pH not higher than 9, more preferably at a pH of 3to 8 (pH 5.5±2.5), although the conditions may vary depending on thetime for the operation.

[0042] The above-mentioned elimination by neutralization is effected bybringing the reaction mixture after completion of the reaction intocontact with a base. In this case, although the base itself maybecontacted with the above reaction mixture, it is a preferred generalpractice to dissolve or suspend the base in water or an organic solventand bring the solution or suspension into contact with the abovereaction mixture.

[0043] The base to be used in the above neutralization is notparticularly restricted but includes, among others, amines such astriethylamine and diisopropylamine; alkali metal hydroxides such assodium hydroxide, potassium hydroxide and lithium hydroxide; alkalineearth metal hydroxides such as calcium hydroxide and magnesiumhydroxide; carbonate salts such as sodium carbonate and potassiumcarbonate; and hydrogen carbonate salts such as sodium hydrogencarbonate and potassium hydrogen carbonate. From the viewpoint ofoperability, carbonate or hydrogen carbonate salts are preferred amongothers, and hydrogen carbonate salts are more preferred. Sodium hydrogencarbonate is still more preferred.

[0044] Meanwhile, with those amino acid derivatives (1) whose aminogroup is protected by benzyloxycarbonyl, for exampleN-benzyloxycarbonyl-S-phenylcysteine (R⁰=hydrogen, R¹=benzyloxycarbonyl,R²=phenyl), there is a tendency toward contamination with a structurallyanalogous impurity, namely a compound represented by the general formula(4):

[0045] wherein R² and * are as defined above, for exampleN-benzyl-S-phenylcysteine, differing from the corresponding derivative(1) only in that the above benzyloxycarbonyl group is replaced by abenzyl group and originating from the raw material benzyloxycarbonylhalide and/or the process of their synthesis (e.g. Schotten-Baumannreaction). It is difficult to completely prevent the formation of thisstructurally analogous impurity (4) and, further, it is very difficultto remove said contaminant for purification even when the amount thereofis small (e.g. 1% or less, or 0.5% or less).

[0046] As is well known in the art, a structurally analogous impurity,because of its similarity in structure, behaves in the same manner asthe substrate compound or desired final compound in the proceduralprocess from reaction to after treatment and as a result tend to existas a contaminant in the final product. When the final product is amedicinal, the presence of a trace amount of a structurally analogousimpurity may cause a very serious problem in some cases.

[0047] According to the process of the present invention, it is possibleto cause the structurally analogous impurity (4) existing in the aminoacid derivative (1) whose amino group is protected with abenzyloxycarbonyl group to remain substantially unreacted. This is avery great advantage of the process of the present invention.

[0048] Specifically, when N-benzyloxycarbonyl-S-phenylcysteine methylester (R⁰=hydrogen, R¹=benzyloxycarbonyl, R²=phenyl, R³=methyl), forinstance, is to be produced as the above cysteine derivative (2) usingN-benzyloxycarbonyl-S-phenylcysteine (R⁰=hydrogen, R¹=benzyloxycarbonyl,R²=phenyl) as the amino acid derivative (1), it is possible to cause thecontaminant N-benzyl-S-phenylcysteine existing as an impurity in theabove amino acid derivative (1) to remain substantially unreacted evenafter completion of the reaction by using a thionyl halide in an amountof about 1 equivalent or slightly larger than 1 equivalent relative tothe above amino acid derivative (1) or a strong acid in an amount of0.01 to 0.1 equivalent, as mentioned hereinabove. Therefore, by makinguse of the large difference in solubility in a solvent, for instance,between the desired product N-benzyloxycarbonyl-S-phenylcysteine methylester and the remaining impurity N-benzyl-S-phenylcysteine, it ispossible to readily remove this impurity from the reaction mixture bycarrying out such a general separation procedure as filtration and/orextraction and washing after completion of the reaction.

[0049] A method of recovering the desired product of the presentinvention, namely the cysteine derivative (2), from the reaction mixtureis now described. The reaction mixture so referred to herein means thereaction mixture after completion of the reaction.

[0050] In the production process of the present invention, the desiredcompound, namely the cysteine derivative (2), may be recovered from thereaction mixture by a separational procedure comprising extraction andwashing or it can be recovered without any extraction procedure. Ofcourse, these recovery procedures are conducted under acidic to weaklybasic conditions. When the conditions are defined in terms of pH, the pHis generally not higher than 10, preferably not higher than 9, morepreferably not higher than 8.

[0051] The method of recovering the above cysteine derivative (2) fromthe reaction mixture by a separational procedure comprising extractionand washing is a method of recovery which comprises carrying outextraction with an organic solvent, separating the organic extractphase, and carrying out the concentration or crystallization. Preferredmodes of this method are described below in detail. It is moreadvantageous to use these preferred modes in combination.

[0052] In the above extraction step, the reaction mixture as such may beblended with an extraction solvent. For preventing the remainingreaction solvent from adversely affecting the procedure for separatingthe organic phase from the aqueous phase, however, blending afterreducing the content of the above reaction solvent is preferred. In thiscase, it is more preferred to replace the reaction solvent with theextraction solvent. In cases where the above alcohol (3), in particulara lower alcohol containing 1 to 4 carbon atoms, which is particularlyhigh in affinity for water, remains in a large amount in the reactionmixture, it is judicious to conduct the separation procedure afterlowering the alcohol content in the system by recovering the alcohol,for instance.

[0053] According to the prior art, the separation of the cysteinederivative (2) by extraction and washing has drawbacks, for example itrequires a large amount of an organic solvent, making it difficult toemploy such method of separation on an industrial scale where theproductivity is an important factor. However, investigations made by thepresent inventors revealed that when the procedure mentioned below isemployed, such separation can be carried out in a simple and efficientmanner and can give high yields.

[0054] The difficulty in phase separation is generally caused by theformation of an emulsified intermediate phase between the organic andaqueous phases and the slow rate of disappearance of that intermediatephase. It was found, however, that the difficulty in phase separation inthe steps of extraction and washing of the desired product of thepresent invention, namely the cysteine derivative (2) is not due to thatgenerally known formation of an emulsified intermediate phase but is dueto failure in forming separable two layers, the upper and lower layers,because, in spite of separation into two phases, an organic and anaqueous phase, a large number of droplets consisting of one phase remaindispersed in the other phase but will not coalesce.

[0055] In the practice of the present invention, it is preferred that,in the step of organic-aqueous phase separation, the mixture composed ofan organic phase containing the above cysteine derivative (2) and anaqueous phase be supplemented with an inorganic salt and/or subjected towarming treatment to thereby successfully attain phase separation intothe organic and aqueous phases, followed by recovering the organic phasecontaining the cysteine derivative (2). In this case, the addition of aninorganic salt is more preferably combined with the warming treatment.For example, when an inorganic salt such as sodium chloride and sodiumsulfate is caused to coexist and at the same time the system ismaintained in a warmed condition, two layers, the upper and lowerlayers, can be formed very easily and, at the same time, separation canbe realized efficiently without using the organic solvent in a largeamount.

[0056] More specifically, when the above cysteine derivative (2) isN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester (R⁰=hydrogen atom,R¹=benzyloxycarbonyl, R²=phenyl, R³=methyl) and the extraction iscarried out using toluene, the mixture is warmed at a temperature of notlower than about 40° C. in the presence of sodium chloride in an amountsuch that its concentration in water amounts to about 5% by weight tothe saturated concentration, whereby two layers composed of the upperand lower layers can be formed very easily.

[0057] The extraction solvent to be used in the above method ofseparation is not particularly restricted but includes, among others,aromatic hydrocarbons such as toluene; acetate esters such as ethylacetate; and ethers such as methyl tert-butyl ether. In particular, theuse of at least one of toluene and ethyl acetate is preferred.

[0058] For removing, by causing it to transfer to the aqueous phase inseparating the organic phase from the aqueous phase, the above compound(4) which has a benzyl group introduced onto the amino group such asN-benzyl-S-phenyl-L-cysteine and remains unreacted as an impurity, it isjudicious to add an inorganic salt and/or perform warming treatment, asmentioned above, and carry out the phase separation procedure includingextraction and washing, if necessary repeatedly, under acidicconditions, for example at a pH of not higher than 3, ordinary nothigher than 2, preferably not higher than 1, more preferably in thevicinity of pH 0 or less than it. According to need, it is also possibleto cause a water-miscible organic solvent (e.g. a monohydric alcoholcontaining 1 to 4 carbon atoms, for example methanol) to be contained inthe aqueous phase.

[0059] In recovering the above cysteine derivative (2) from thethus-separated organic phase containing the cysteine derivative (2), themethod mentioned below can be employed by which the recovery can beaccomplished in a simple manner with high yields even in commercialscale production where the productivity is of importance.

[0060] The present inventors found that, in distilling off the solventfrom the organic phase containing the above cysteine derivative (2), itis important to select the operational conditions taking intoconsideration the melting temperature of the above cysteine derivative(2) or the melting temperature of the concentrate mainly comprising theabove cysteine derivative (2) and the temperature for distilling off thesolvent. Namely, it was unexpectedly found that by distilling off theabove solvent at a temperature above such a melting temperature (wherethere is a melting temperature range, above the temperature at which themelting begins), it is possible to obtain the above cysteine derivative(2) as an oil very low in viscosity and easy to handle.

[0061] When the above cysteine derivative (2) isN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester (R⁰=hydrogen atom,R¹=benzyloxycarbonyl, R²=phenyl, R³=methyl), for instance, theN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester can be recovered asan oil, by distilling off the toluene at about 60 to 70° C. or above,from the organic phase obtained by toluene extraction, washing and phaseseparation.

[0062] The above oily cysteine derivative (2) may be recovered in theform of a solution by adding a solvent, if necessary. Whentetrahydrofuran, for instance, is added to the above oily product, thederivative (2) can be recovered in the form of a tetrahydrofuransolution. By doing so, a solvent differing much in boiling point, forexample toluene, which is a high-boiling solvent, can be exchanged fortetrahydrofuran, which is a low-boiling solvent, very efficiently.

[0063] When the above solvent removal by distillation is carried out ata lower temperature, the above cysteine derivative (2) solidifies duringdistilling off the solvent, increasing the load on stirring or causinginsufficient stirring, which results in incomplete solvent removal bydistillation. In this case, the derivative (2) can be obtained in theform of a solution only by a complicated procedure comprising repeatingthe procedure for solvent removal by distillation while supplementingthe solvent. For example, the replacing a high-boiling toluene solutionto a low-boiling tetrahydrofuran solution is very inefficient and astabilizer such as BHT coexisting in tetrahydrofuran is concentrated toa high level in the solution and may sometimes produce unfavorableaffects when used in the next step.

[0064] An alternative method of recovering the above cysteine derivative(2) from the above organic phase resulting from phase separationcomprises causing the above cysteine derivative (2) to precipitate outfrom the organic phase containing the cysteine derivative (2) and anorganic solvent miscible with an aliphatic hydrocarbon or from aconcentrate derived from said organic phase by using such an aliphatichydrocarbon as a poor solvent, if necessary with cooling, followed byrecovering the precipitate, for example a crystallization procedure.

[0065] When, for example, the cysteine derivative (2) is one in which R⁰is a hydrogen atom, R¹ is a urethane-type protective group, R² is aphenyl group and R³ is an alkyl group containing 1 to 4 carbon atoms, itis possible to favorably crystallize out the above cysteine derivative(2) from its solution in an organic solvent miscible with an aliphatichydrocarbon by using such an aliphatic hydrocarbon as a poor solvent, ifnecessary with cooling and/or concentration.

[0066] The organic solvent miscible with an aliphatic hydrocarbon is notparticularly restricted but includes aromatic hydrocarbons such astoluene; acetate esters such as ethyl acetate and isopropyl acetate; andethers such as methyl tert-butyl ether. Toluene is particularlypreferred among the aromatic hydrocarbons, ethyl acetate among theacetate esters, and methyl tert-butyl ether among the ethers. At leastone of these solvents is preferably used. The aliphatic hydrocarbon tobe used as a poor solvent in combination with the above organic solventis not particularly restricted but may be, for example, hexane, heptane,methylcyclohexane or the like. Hexane is preferred, however.

[0067] Generally, it is convenient to admix the above poor solvent withan organic solvent solution containing the above cysteine derivative(2). For controlling the amount of the cysteine derivative remainingdissolved in the step of crystal precipitation, it is also possible, ifnecessary, to cool the mixture, or to concentrate the above solution bydistilling off under reduced pressure an arbitrary amount of the solventconstituting the above solution, followed by admixing with the poorsolvent. It is judicious to adjust the amount of the poor solvent to beadded, the amount of the above solvent to be distilled off, and/or theextent of cooling, as necessary, depending on the solvent speciesconstituting the above solution and the amount thereof.

[0068] The amount of the above poor solvent is not particularlyrestricted. From the viewpoint of improved recovery rate and improvedoperability, among others, it is generally about 10% by weight or more,preferably about 20% by weight or more, more preferably about 30% byweight or more, based on the total weight of the solution, for instance.

[0069] The above method of crystallization can be carried out bycombinedly using generally known crystallization procedures such ascrystallization with cooling or by concentration.

[0070] Generally, it is preferred that the concentration in the step ofcrystallization be not more than about 30% (w/v), more preferably notmore than about 20% (w/v), as the weight of the cysteine derivative (2)relative to the total volume of the solution as resulting from adjustingthe amount of the poor solvent to be added and the amount of the solventto be distilled off, among others, within the ranges which allow thesolution to maintain its fluidity.

[0071] The temperature for the above precipitation is not restricted butgenerally is from the solidifying temperature to about 60° C. Forproducing a crystallization mixture (slurry) having good properties, itis judicious to allow the crystallization to proceed slowly, for exampleby slow cooling or slow addition of the above poor solvent.

[0072] For preventing the cysteine derivative (2) from being decomposedor racemized, the above method of crystallization is preferably carriedout under acidic to weakly basic conditions after removing theunnecessary base component.

[0073] In the above method of crystallization, the cysteine derivativeis precipitated as crystals from a solution of the cysteine derivative(2) in an organic solvent miscible with an aliphatic hydrocarbon usingthe aliphatic hydrocarbon as a poor solvent, if necessary with coolingand/or carrying out concentration, while removing the impurity mixed inthe cysteine derivative therefrom.

[0074] The above-mentioned method of crystallization is applicable alsoto the crystallization of the product obtained by a process other thanthe production process of the present invention, for example by theprocess comprising converting the hydroxyl group of a serine derivativeto a leaving group and then carrying out the substitution reaction[Tetrahedron Lett., vol. 28, page 6069 (1987); ibid., vol. 34, page 6607(1993); EP 604185 Al; WO 98/30538]. Particularly when a low grade of thecysteine derivative (2) containing impurities such as the optical isomerof the cysteine derivative (2) and a dehydroalanine derivative istreated by the above-mentioned method of crystallization, saiddehydroalanine derivative being represented by the general formula (5):

[0075] wherein R⁰ represents a hydrogen atom, R¹ represents a urethanetype amino-protecting group and R³ represents an alkyl group containing1 to 4 carbon atoms,

[0076] and being hardly formed as byproducts in the production processof the present invention,

[0077] the impurities mentioned above can be removed efficiently, hencethe above method of crystallization is effective also as a method ofpurifying the cysteine derivative (2). For example, the content of theimpurity optical isomer can be reduced to 2% or less, preferably to 1%or less, and the dehydroalanine derivative (5) can be almost completelyremoved.

[0078] For the effects of the above method of purification to beproduced to its maximum, when the cysteine derivative (2) contains itsoptical isomer as an impurity, it is judicious to adjust thecrystallization conditions such as species of the solvent, the mixingratio therebetween and the recovery temperature in the step ofcrystallization so that the relative equilibrium solubility (equilibriumratio between the amounts dissolved) in the crystallization mixturebetween the optically active cysteine derivative (2), which is thedesired product, and its optical isomer may become lower than theoptical purity of the cysteine derivative before crystallization, tothereby cause preferential crystallization of the desired opticallyactive cysteine derivative (2), which exists in excess, whileefficiently removing the impurity optical isomer into the mother liquor.By this measure, it is possible to obtain the optically active cysteinederivative (2) with a percent existence of the desired optical isomer ofnot less than 98%, preferably not less than 99%.

[0079] In other words, the relative solubility in the solution betweenthe optically active cysteine derivative (2) and its optical isomer hasa constant value (namely, there is an equilibrium ratio between theamounts dissolved) depending on the crystallization conditions selected,such as the solvent species and the mixing ratio between the solventsconstituting the mixed solvent, independently of the optical purity ofthe cysteine derivative used in the crystallization step, so that whenthe optical purity of the cysteine derivative used in thecrystallization step is higher than the equilibrium ratio between thedissolved amounts under the crystallization conditions selected, it ispossible to cause preferential crystallization of one of the opticalisomers which exists in excess.

[0080] The crystals of the optically active cysteine derivative (2)obtained in that manner can be recovered by a conventional solid-liquidseparation procedure such as pressure filtration, filtration underreduced pressure and centrifugation and, further, the solvent can beremoved by atmospheric drying or drying under reduced pressure (vacuumdrying).

[0081] On the other hand, a method of recovering the product cysteinederivative (2) without any extraction procedure comprises recovering theabove cysteine derivative (2) from an alcohol, water, or a mixturethereof, for example by crystallization, without performing anyextraction procedure. The alcohol to be used in this method is notrestricted to the alcohol (3) used in carrying out the reaction but maybe an alcohol added after reaction and different from the above alcohol(3). Such a method of recovery without performing any extractionprocedure is advantageous since the percentage recovery can be readilyimproved by causing water to exist. For that purpose, the solubilityand/or the mixing ratio between water and a solvent such as an alcoholin the step of recovery is controlled by admixing water with thereaction mixture, distilling off under reduced pressure an arbitrarilyselected amount of the above alcohol (3) and/or some other solvent inthe reaction mixture, or by further adding water.

[0082] Depending on the species and amount of the solvent used in thereaction, such as an alcohol, the recovery is preferably carried out,according to need, by a combination of water content adjustment, solventremoval by distillation, concentration, and cooling.

[0083] In increasing the water content in the reaction mixture when thereaction solvent used is a highly water-miscible monohydric alcoholcontaining 1 to 4 carbon atoms, such as methanol, the water content inthe system in the step of recovery is generally not less than 10% byweight, preferably not less than about 20% by weight, more preferablynot less than about 30% by weight, based on the total amount of theabove alcohol (3) and water, from the improved recovery and improvedoperability viewpoint.

[0084] The reaction mixture contains not only the desired productcysteine derivative (2) but also the strong acid, or the byproducthydrogen halide and sulfur dioxide when a thionyl halide is used, and isthus in a state containing very strongly acidic substances. It is notalways necessary to remove these strongly acidic substances prior to thestep of recovery. When they are removed, however, they can be removed bythe method mentioned hereinabove. Such removal is effected byneutralization and, when the neutralization is carried out in the mannermentioned hereinabove, the precipitation can be effected while allowingthe water used together with the base and the water formed to coexist asthey are. In the presence of water, the contaminants such as theunreacted raw materials and salts formed therefrom generally show highsolubilities while the desired product cysteine derivative (2) shows alow solubility. Therefore, there is a tendency toward efficientprecipitation of the cysteine derivative (2) with high quality.

[0085] When, in the cysteine derivative (2), R⁰ is a hydrogen atom, R¹is a urethane type protective group, R² is a phenyl group and R³ is analkyl group containing 1 to 4 carbon atoms, the above cysteinederivative (2) can favorably be crystallized out by using, as thecrystallization solvent, a monohydric alcohol containing 1 to 4 carbonatoms or a mixed solvent composed of a monohydric alcohol containing 1to 4 carbon atoms and water under acidic to weakly basic conditions.Particularly when the above cysteine derivative (2) isN-benzyloxycarbonyl-S-phenylcysteine methyl ester (R⁰=hydrogen atom,R¹=benzyloxycarbonyl, R²=phenyl, R³=methyl), the use, as thecrystallization solvent, of methanol or a mixed solvent composed ofmethanol and water under acidic to weakly basic conditions isparticularly preferred. In the case of a mixed solvent composed ofmethanol and water, the content of water in the system, which isrecommendable from the viewpoint of impurity removal, is generally lessthan about 10% by weight, preferably less than about 5% by weight, morepreferably less than about 1% by weight, based on the total amount ofmethanol and water. It is judicious to use methanol singly as thecrystallization solvent, since, in that case, simplification of thecrystallization procedure and/or facilitation of recovery and reuse ofthe solvent, in addition to efficient removal of said impurity, can beattempted.

[0086] When a mixed solvent composed of a monohydric alcohol containing1 to 4 carbon atoms and water is used as the crystallization solvent,the condition concerning the water content in the system which isadequate for the purpose in the practical procedure may be determinedtaking into consideration the recovery rate, the operability and theeffect of removing each impurity.

[0087] The above method of crystallization is applicable also to thecrystallization of the product obtained by a process other than theproduction process of the present invention. In particular when a lowgrade of the cysteine derivative (2) containing impurities such as theoptical isomer and a dehydroalanine derivative is subjected to theabove-mentioned method of crystallization,

[0088] said dehydroalanine derivative being represented by the generalformula (5):

[0089] wherein R⁰ represents a hydrogen atom, R¹ represents a urethanetype amino-protecting group and R³ represents an alkyl group containing1 to 4 carbon atoms,

[0090] and being hardly formed as byproducts in the production processof the present invention,

[0091] the impurities mentioned above can be removed efficiently, hencethe above method of crystallization is effective also as a method ofpurifying the cysteine derivative (2). For example, the content of theimpurity optical isomer can be reduced to 2% or less, preferably to 1%or less, and the dehydroalanine derivative (5) can be almost completelyeliminated.

[0092] For the effects of the above method of purification to beproduced to its maximum, when the cysteine derivative (2) contains itsoptical isomer as an impurity, it is judicious to adjust thecrystallization conditions such as the species of solvents, the mixingratio therebetween and the recovery temperature in the step ofcrystallization so that the relative equilibrium solubility (equilibriumratio between the amounts dissolved) in the crystallization mixturebetween the desired optically active cysteine derivative (2) and itsoptical isomer may become lower than the optical purity of the cysteinederivative before crystallization, to thereby cause preferentialcrystallization of the desired optically active cysteine derivative (2),which exists in excess, while efficiently removing the impurity opticalisomer into the mother liquor. By this measure, it is possible to obtainthe optically active cysteine derivative (2) with a percent existence ofthe desired optical isomer of not less than 98%, preferably not lessthan 99%.

[0093] In other words, the relative solubility in the solution betweenthe optically active cysteine derivative (2) and its optical isomer hasa constant value (namely, there is an equilibrium ratio between theamounts dissolved) depending on the crystallization conditions selected,such as the solvent species and the mixing ratio between the solventsconstituting the mixed solvent, independently of the optical purity ofthe cysteine derivative used in the crystallization step, so that whenthe optical purity of the cysteine derivative used in thecrystallization step is higher than the equilibrium ratio between thedissolved amounts under the crystallization conditions selected, it ispossible to cause preferential crystallization of one of the opticalisomers which exists in excess.

[0094] The temperature for the above crystallization is not restrictedbut generally is from the solidifying temperature to about 60° C.

[0095] In particular, for recovering the cysteine derivatives (2) ascrystals, it is judicious to adjust the concentration of the same in thestep of recovering to a slurry concentration of not higher than 30%(w/v), preferably not higher than 20% (w/v) by adjusting the amount ofwater to be admixed, the amount of the solvent to be distilled off andthe recovery temperature, among others, to thereby maintain the fluidityin the step of crystallization and to obtain a crystallization mixtureshowing good slurry properties by slowly cooling or slowly admixingwater, for instance.

[0096] In the above step of crystallization, generally known methods ofcrystallization such as cooling crystallization or by concentratingcrystallization can be employed. The so-called reactive crystallizationis also considered possible which comprises causing the cysteinederivative (2) formed with the progress of the reaction to crystallizeout successively by adjusting the conditions such as the temperature,concentration and other factors for the reaction.

[0097] The cysteine derivative (2) that has crystallized out can becollected by a conventional solid-liquid separation procedure such aspressure filtration, filtration under reduced pressure andcentrifugation and, further, the solvent can be removed by atmosphericdrying or drying under reduced pressure (vacuum drying).

[0098] The present invention thus provides an industrially excellentmethod by which the side reactions are prevented and the cysteinederivative (2) is obtained in a simple and efficient manner. The yieldof the cysteine derivative (2) can be expected to be not less than 90%,preferably almost quantitative. A particular advantage of the presentinvention is that the racemization can favorably be prevented. Thedegree of racemization, namely the decrease in optical purity [in thecase of L form, for instance, 100%×L form/ (L form+D form)], can besuppressed to 2% or less, preferably 1% or less.

BEST MODES FOR CARRYING OUT THE INVENTION

[0099] The following examples illustrate the present invention infurther detail. They are, however, by no means limitative of the scopeof the invention.

[0100] In the examples, the optical purity [100%×L-form/ (L form+Dform)] of the N-benzyloxycarbonyl-S-phenyl-L-cysteine used was 99.6%.The optical purity [100%×L-form/ (L form+D form)] of the productN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester was determinedusing a HPLC column for separating optical isomers (CHIRALPAK AD,product of Daicel Chemical Industry). The yield and purity ofN-benzyloxycarbonyl-S-phenyl-L-cysteine were given for the sum of theL-form and D-form.

EXAMPLE 1

[0101] In a nitrogen atmosphere, 1,500 ml of methanol was added to 300.9g of N-benzyloxycarbonyl-S-phenyl-L-cysteine and 8.64 g ofp-toluenesulfonic acid monohydrate, and the reaction was allowed toproceed under reflux. After 6 hours of reaction, the mixture was cooledto about 40° C., and the pH was adjusted to 5.5 by adding 82 ml of a 5%(by weight) aqueous solution of sodium hydrogen carbonate. Then, themethanol was replaced with toluene at about 40 ° C., to give 1152.7 g ofa toluene-substituted reaction mixture. Then, 200 ml of a 20% (byweight) aqueous solution of sodium chloride was added at 45 to 50° C.,and the pH was adjusted to 7.8 with 23 ml of a 5% (by weight) aqueoussolution of sodium hydrogen carbonate. After separation of the aqueousphase, the organic phase was washed with two 200-ml portions of a 20%(by weight) aqueous solution of sodium chloride at 45 to 50° C., to give1130.6 g of a toluene solution, This toluene solution contained 309.8 g(concentration: 27.4% by weight) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; the yield was 99%and the optical purity was 99.7%.

[0102] Further, from 1129.1 g of the toluene solution containing 309.4 gof N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester, removal of thetoluene by distillation was started at an inside temperature of 60 to70° C. and, finally, at an internal pressure of 20 mm Hg and an insidetemperature of 68° C., there was obtained 317.2 g of an oil containingN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester.

[0103] Prior to the above toluene removal by distillation, melting pointdetermination was carried out and it was confirmed that crystals of theN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester had a melting pointof 65° C. Referring to the melting point 65° C., a temperature of 65 to70° C. was employed as the operation temperature. In the subsequentexamples, the operation temperature was selected in the same manner.

[0104] To the above oil was added 298.6 g of tetrahydrofuran, to give615.8 g of a tetrahydrofuran solution ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester. Thistetrahydrofuran solution contained 309.0 g (concentration: 50.2% byweight) of N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; theyield was 100%, the optical purity was 99.7% and the amount of residualtoluene was 1.9 g (concentration: 0.3% by weight). The formation ofN-benzyloxy-carbonyldehydroalanine methyl ester was not observed.

Reference Example 1

[0105] About 150 g of the reaction mixture after 6 hours of reaction asobtained by the same procedure as in Example 1 and having anN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester concentration of20.5% by weight was adjusted to pH 6, 10, 11 or 12 by adding an aqueoussolution of sodium hydrogen carbonate or an aqueous solution of sodiumhydroxide, and each mixture was maintained at 40° C. for 1 hour withstirring. Each reaction mixture was subjected to substitution of themethanol for toluene in the same manner as in Example 1 at about 40° C.and 20 ml of a 20% (by weight) aqueous solution of sodium chloride wasadded to the toluene-substituted reaction mixture. After separation ofthe aqueous phase, the organic phase was washed with two 20-ml portionsof a 20% (by weight) aqueous solution of sodium chloride at 45 to 50°C., and the toluene solution obtained was assayed for the optical purityof N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester and the yield ofN-benzyloxycarbonyldehydroalanine methyl ester. The results thusobtained are shown in Table 1. TABLE 1 Opitcal Yield ofN-benzyloxycarbonyl- purity dehydroalanine methyl ester pH (%) (mol %) 6 99.6 Not detected 10 99.6 0.2 11 80.4 2.6 12 73.1 2.1

EXAMPLE 2

[0106] In a nitrogen atmosphere, a solution prepared by admixing 458 mgof concentrated sulfuric acid with 10 ml of methanol was added to 30.03g of N-benzyloxycarbonyl-S-phenyl-L-cysteine and 140 ml of methanol, andthe reaction was allowed to proceed under reflux. After 10 hours ofreaction, the mixture was cooled to room temperature and the pH wasadjusted to 5.5 by adding 8 ml of a 5% (by weight) aqueous solution ofsodium hydrogen carbonate. Then, the methanol was replaced with tolueneat about 50° C., to give 116 g of a toluene-substituted reactionmixture. The mixture was then washed with two 40-ml portions of a 10%(by weight) aqueous solution of sodium sulfate at 45 to 50° C. to give113.5 g of a toluene solution. This toluene solution contained 30.7 g(concentration: 27.1% by weight) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; the yield was 98%and the optical purity was 99.6%.

[0107] Further, removal of the toluene by distillation was started from112.0 g of the toluene solution containing 30.3 g ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester at an insidetemperature of 60 to 70° C. and, finally, at an internal pressure of 20mm Hg and an inside temperature of 67° C., there was obtained 31.5 g ofan oil containing N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester.To this was added 30.0 g of tetrahydrofuran, to give 61.5 g of atetrahydrofuran solution of N-benzyloxycarbonyl-S-phenyl-L-cysteinemethyl ester. This tetrahydrofuran solution contained 30.3 g(concentration: 49.2% by weight) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; the yield was100%, the optical purity was 99.6% and the amount of residual toluenewas 0.2 g (concentration: 0.3% by weight). The formation ofN-benzyloxycarbonyldehydroalanine methyl ester was not detected.

EXAMPLE 3

[0108] In a nitrogen atmosphere, 11.8 g of thionyl chloride was added to47 ml of methanol over 1 hour with stirring at −3 to 0° C., and thereaction was further allowed to proceed for 1 hour at the sametemperature (solution A). Separately, 30.02 g ofN-benzyloxycarbonyl-S-phenyl-L-cysteine was admixed with 103 ml ofmethanol (solution B). Solution A was added to solution B over 20minutes at about 25° C. and then the reaction was allowed to proceed atthe same temperature for 2 hours. Thereafter, the mixture was warmed toabout 50° C. While effecting degassing and distilling off the methanolunder reduced pressure at an internal pressure of 100 mm Hg, toluene wasadded, to give 121.5 g of a toluene solution. This solution was adjustedto pH 8.1 by adding 20 ml of a 10% (by weight) aqueous solution ofsodium chloride and 10 ml of a 5% (by weight) aqueous solution of sodiumhydrogen carbonate at 40° C. After separation of the aqueous phase, theorganic phase was washed with two 20-ml portions of a 10% (by weight)aqueous solution of sodium chloride at 40 to 45° C., to give 123.0 g ofa toluene solution. This toluene solution contained 30.7 g(concentration: 24.9% by weight) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; the yield was 98%and the optical purity was 99.6%. The formation ofN-benzyloxycarbonyldehydroalanine methyl ester was not detected.

EXAMPLE 4 Reference Example 2

[0109] In a nitrogen atmosphere, 150 ml of methanol was added to 30.01 gof N-benzyloxycarbonyl-S-phenyl-L-cysteine and 864 mg ofp-toluenesulfonic acid monohydrate, and the reaction was allowed toproceed under reflux. After 6 reaction hours, the mixture was cooled toabout 50° C., and the pH was adjusted to 5.5 by adding 8 ml of a 5% (byweight) aqueous solution of sodium hydrogen carbonate. Then, themethanol was replaced with toluene at about 50° C., to give 103.87 g ofa toluene-substituted reaction mixture containing 30.97 g(concentration: 29.8% by weight; yield: 99%; optical purity: 99.7%) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester (Example 4). Then,15 ml of pure water was added to this toluene-substituted reactionmixture at room temperature, and the mixture was stirred for 15 minutesand then allowed to stand, whereupon the aqueous phase formed a largenumber of droplets, which remained dispersed or settled in the organicphase but did not unite with one another. This procedure thus failed toform two layers, namely the upper and lower layers, separable from eachother, hence to enable phase separation (Reference Example 2).

[0110] To a mixture of the above toluene solution and water was added335 mg (corresponding to a concentration in water of 6.3% by weight) ofsodium chloride, and the mixture was stirred for 15 minutes at 45 to 50°C. and then allowed to stand, whereupon the organic and aqueous phasesrapidly formed two separable layers, upper and lower layers; phaseseparation thus became possible (Example 4).

EXAMPLE 5 Reference Example 3

[0111] Toluene removal by distillation was started from 254.4 g of atoluene solution obtained in the same manner as in Example 1 andcontaining 49.4 g (concentration: 19.4% by weight; optical purity:99.7%) of N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester at aninside temperature of 60 to 70° C. and, finally, 50.6 g of an oilcontaining N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester wasobtained at an internal pressure of 20 mm Hg and an inside temperatureof 70° C. To this was promptly added 43.4 g of tetrahydrofuran, to give93.9 g of a tetrahydrofuran solution ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester. Thistetrahydrofuran solution contained 49.3 g (concentration: 52.5% byweight) of N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; theyield was 100%, the optical purity was 99.7% and the amount of residualtoluene was 0.7 g (concentration: 0.3% by weight) (Example 5).

[0112] Separately, from 254.4 g of the same toluene solution containing49.4 g (concentration: 19.4% by weight; optical purity: 99.7%) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester was distilled offthe toluene at an inside temperature of 40° C., whereupon stirringbecame more and more difficult and, when the concentration ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester reached 78.1% byweight, the stirrer stopped (Reference Example 3).

EXAMPLE 6

[0113] A toluene solution (58.3 g) obtained in the same manner as inExample 1 and containing 13.0 g (concentration: 22.3% by weight) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester was concentratedunder reduced pressure at an inside temperature of 40° C. to finallygive a toluene solution with an N-benzyloxycarbonyl-S-phenyl-L-cysteinemethyl ester concentration of 65% by weight. Dropwise addition of hexaneto this solution was started at 40° C. with stirring while the mixturewas slowly cooled to 20° C., to thereby cause precipitation of whitecrystals. After addition of a total amount of 130 ml of hexane, themixture was cooled to −10° C. and further stirred for 3 hours. Then, thecrystals were recovered by filtration and dried under reduced pressure(1 to 30 mm Hg, 20 to 40° C., 10 hours). The crystals obtained weighed12.5 g (yield 95%), the purity was 99.5% by weight and the opticalpurity was 99.8%.

[0114] The thus-obtained N-benzyloxycarbonyl-S-phenyl-L-cysteine methylester gave the following 400 MHz nuclear magnetic resonance spectrum(CDCl₃; internal standard: TMS): δ: 3.36-3.45 (2H, m), 3.53 (3H, s),4.61-4.65 (1H, m), 5.03-5.10 (2H, ABq, J=12.5 Hz), 5.60-5.62 (1H, b),7.17-7.45 (10H, m).

Reference Example 4

[0115] In a nitrogen atmosphere and at room temperature, 125 mg ofsodium hydride (content: 67.4%) was added to a solution composed of 399mg of thiophenol and 3 ml of N,N-dimethylformamide, and the mixture wasstirred for 30 minutes, and the resulting solution was maintained at 20°C. To the solution was added 971 mg ofN-benzyloxycarbonyl-O-mesyl-L-serine methyl ester, followed by washingthe residue thereof into the solution with 2 ml ofN,N-dimethylformamide. After the lapse of 2 hours, a minimum amount ofthe reaction mixture was taken out and analyzed by HPLC; the startingmaterial N-benzyloxycarbonyl-O-mesyl-L-serine methyl ester was no moredetected, the ratio between the total number of moles ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester andN-benzyloxycarbonyl-S-phenyl-D-cysteine methyl ester and the number ofmoles of N-benzyloxycarbonyldehydroalanine methyl ester was 89:11, andthe optical purity was 87.7%. After a total 20 hours of reaction, thereaction mixture obtained was analyzed by HPLC. As a result, the yieldof N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester was 696 mg (69%yield) and the optical purity was 80.1%. The yield ofN-benzyloxycarbonyldehydroalanine methyl ester was 2.2%.

Reference Example 5

[0116] In a nitrogen atmosphere and at room temperature, 52 ml of 1 Naqueous sodium hydride and 32 ml of distilled water were added to 5.87 gof thiophenol with stirring and the resulting mixture was stirred for 30minutes. Then, 0.74 g of benzyltributylammonium chloride was added, andthe solution was cooled to about 10° C. To the solution was added, allat one, a solution composed of 13.9 g of N-carbobenzoxy-O-mesyl-L-serineand 60 ml of ethyl acetate, with the residual portion being washed with10 ml of ethyl acetate. After the reaction was allowed to proceed at 10°C. for 10 hours, the temperature was raised to room temperature over 2hours. To this reaction mixture were added 70 ml of toluene and 8.2 g(corresponding to a concentration in water of 10% by weight) of sodiumchloride for one extraction procedure, which was carried out at 45˜50°C. After separation of the aqueous layer, the extract was washed withtwo 50-ml portions of a 10% (by weight) aqueous solution of sodiumchloride at 45 to 50° C. to give 136.8 g of an extract. This extractcontained 13.8 g of N-carbobenzoxy-S-phenyl-L-cysteine methyl ester and0.2 g of N-benzyloxycarbonyldehydroalanine methyl ester; the yield was95% and the optical purity was 94.5%.

EXAMPLE 7

[0117] In a nitrogen atmosphere, a solution prepared by admixing 65 mgof concentrated sulfuric acid with 10 ml of methanol was added to 20.08g of N-benzyloxycarbonyl-S-phenyl-L-cysteine and 40 ml of methanol, andthe reaction was allowed to proceed under reflux. After 50 hours ofreaction, the mixture was cooled to about 25° C. The pH of the solutionwas 1.2. Then, the solution was cooled to −15° C. over 3 hours withvigorous stirring, and the resulting crystalline precipitate wasfiltered off and dried under reduced pressure to give 19.22 g (yield:91%) of white crystals. The purity of these crystals ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester was 99.1% by weightand the optical purity was 99.9%. The formation ofN-benzyloxycarbonyldehydroalanine methyl ester was not detected.

EXAMPLE 8

[0118] In a nitrogen atmosphere, 150 ml of methanol was added to 30.09 gof N-benzyloxycarbonyl-S-phenyl-L-cysteine and 864 mg ofp-toluenesulfonic acid monohydrate, and the reaction was allowed toproceed under reflux. After 6 hours of reaction, the mixture was cooledto about 50° C. and adjusted to pH 8.0 by adding 10 ml of a 5% (byweight) aqueous solution of sodium hydrogen carbonate. Then, 110 ml ofpure water was added dropwise over 3 hours with vigorous stirring atabout 50° C., the resulting mixture was cooled to 5° C., and theresulting precipitate crystals were filtered off and washed in sequencewith a mixed solution composed of 50 ml of methanol and 50 ml of purewater and with 100 ml of pure water and then dried under reducedpressure to give 30.89 g (yield: 98%) of white crystals. With thesecrystals, the purity of N-benzyloxycarbonyl-S-phenyl-L-cysteine methylester was 99.7% by weight and the optical purity was 99.8%. Theformation of N-benzyloxycarbonyldehydroalanine methyl ester was notdetected.

[0119] The thus-obtained N-benzyloxycarbonyl-S-phenyl-L-cysteine methylester gave the following 400 MHz nuclear magnetic resonance spectrum(CDCl₃; internal standard: TMS) δ: 3.36-3.45 (2H, m), 3.53 (3H, s),4.61-4.65 (1H, m), 5.03-5.10 (2H, ABq, J=12.5 Hz), 5.60-5.62 (1H, b),7.17-7.45 (10H, m)

EXAMPLE 9

[0120] In a nitrogen atmosphere, a solution prepared by admixing 458 mlof concentrated sulfuric acid with 10 ml of methanol was added to 30.03g of N-benzyloxycarbonyl-S-phenyl-L-cysteine and 140 ml of methanol, andthe reaction was allowed to proceed under reflux. After 6 hours ofreaction, the mixture was cooled to about 50° C. and adjusted to pH 8.0by adding 15 ml of a 5% (by weight) aqueous solution of sodium hydrogencarbonate. Then, 105 ml of pure water was added dropwise over 3 hourswith vigorous stirring at about 50° C., the resulting mixture was cooledto 5° C., and the precipitate crystals were then filtered off, washed insequence with a mixed solution composed of 50 ml of methanol and 50 mlof pure water and with 100 ml of pure water, and dried under reducedpressure to give 30.67 g (yield: 98%) of white crystals, With thesecrystals, the purity of N-benzyloxycarbonyl-S-phenyl-L-cysteine methylester was 99.6% by weight and the optical purity was 99.8%. Theformation of N-benzyloxycarbonyldehydroalanine methyl ester was notdetected.

EXAMPLE 10

[0121] In a nitrogen atmosphere, 11.8 g of thionyl chloride was added to47 ml of methanol over 1 hour with stirring at 3 to 6° C., and thereaction was further allowed to proceed for 1 hour at the sametemperature (solution A) Separately, in a nitrogen atmosphere, 30.02 gof N-benzyloxycarbonyl-S-phenyl-L-cysteine was admixed with 103 ml ofmethanol (solution B). Solution A was added to solution B over 30minutes at about 25° C. and then the reaction was allowed to proceed atthe same temperature for 2 hours to give 155.2 g of a methanolicsolution. This solution was warmed to about 50° C. and adjusted to pH8.0 by adding 70 ml of a 10% (by weight) aqueous solution of sodiumhydroxide. Then, 57 ml of pure water was added dropwise over 3 hourswith vigorous stirring at about 50° C., the mixture was cooled to 5° C.,and the resulting precipitate crystals were filtered off, washed insequence with a mixed solution composed of 50 ml of methanol and 50 mlof pure water and with 100 ml of pure water and then dried under reducedpressure to give 30.91 g (yield: 98%) of white crystals. With thesecrystals, the purity of N-benzyloxycarbonyl-S-phenyl-L-cysteine methylester was 99.6% and the optical purity was 99.8%. The formation ofN-benzyloxycarbonyldehydroalanine methyl ester was not detected.

EXAMPLE 11

[0122] In a nitrogen atmosphere, 5 ml of methanol and 150 ml of toluenewere added to 30.0 g of N-benzyloxycarbonyl-S-phenyl-L-cysteine and 864mg of p-toluenesulfonic acid monohydrate, and the reaction was allowedto proceed under reflux. While the byproduct water was removed byazeotropic distillation and the toluene phase of the distillate wasreturned to the reaction system, the reaction was allowed to proceed for6 hours. The mixture was then cooled to about 40° C. and adjusted to pH5.5 by adding 8 ml of a 5% (by weight) aqueous solution of sodiumhydrogen carbonate. Then, the methanol was replaced with toluene atabout 40° C. to give 115.3 g of a toluene-substituted reaction mixture.Thereto was then added 20 ml of a 20% (by weight) aqueous solution ofsodium chloride at 45 to 50° C., and the pH was adjusted to 7.9 with 2.3ml of a 5% (by weight) aqueous solution of sodium hydrogen carbonate.After separation of the aqueous phase, the organic phase was washed withtwo 20-mi portions of a 20% (by weight) aqueous solution of sodiumchloride at 45 to 50° C., to give 113.2 g of a toluene solution. Thistoluene solution contained 30.4 g (concentration: 26.9% by weight) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; the yield was 97%and the optical purity was 99.6%.

[0123] Further, from 112.1 g of the toluene solution containing 30.2 gof N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester, removal of thetoluene by distillation was started at an inside temperature of 60 to70° C. and, finally, at an internal pressure of 20 mm Hg and an insidetemperature of 68° C., there was obtained 30.7 g of an oil containingN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester. To this was added29.6 g of tetrahydrofuran, to give 60.9 g of a tetrahydrofuran solutionof N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester. Thistetrahydrofuran solution contained 30.4 g (concentration: 50.4% byweight) of N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; theyield was 100%, the optical purity was 99.6% and the amount of residualtoluene was 0.2 g (concentration: 0.3% by weight).

EXAMPLE 12

[0124] In a nitrogen atmosphere, 150 ml of methanol was added to 30.0 gof N-benzyloxycarbonyl-S-phenyl-L-cysteine containing 0.1 g (0.4% byweight) of N-benzyl-S-phenylcysteine and 864 mg of p-toluenesulfonicacid monohydrate, and the reaction was allowed to proceed under refluxfor 6 hours. The reaction mixture was cooled to about 40° C., to give149.5 g of a methanol solution. This methanol solution contained 30.9 g(concentration: 20.7% by weight) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; the yield was 99%and the optical purity was 99.6%. The solution contained 0.1 g ofN-benzyl-S-phenylcysteine but the formation of N-benzyl-S-phenylcysteinemethyl ester was not detected.

[0125] The methanol solution (147.2 g) containing 30.5 g ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester was adjusted to pH5.5 by adding 8 ml of a 5% (by weight) aqueous solution of sodiumhydrogen carbonate and then the methanol was replaced with toluene atabout 40° C., to give 112.8 g of a toluene-substituted reaction mixture.Then, 10 ml of a 10% (by weight) aqueous solution of sodium chloride wasadded at 45 to 50° C., the pH was adjusted to 0.3 with 35% (by weight)hydrochloric acid, and washing was repeated three times under acidicconditions. Further, 10 ml of a 10% (by weight) aqueous solution ofsodium chloride was added at 45 to 50° C. and the pH was adjusted to 7.9with 19.8 ml of a 5% (by weight) aqueous solution of sodium hydrogencarbonate. After separating the aqueous phase, the organic phase waswashed with two 20-ml portions of a 20% (by weight) aqueous solution ofsodium chloride at 45 to 50° C., to give 111.5 g of a toluene solution.This toluene solution contained 30.4 g (concentration: 27.3% by weight)of N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; the yield was100% and the optical purity was 99.6%. The contamination withN-benzyl-S-phenylcysteine or N-benzyl-S-phenylcysteine methyl ester wasnot detected.

[0126] Further, from 109.1 g of the toluene solution containing 29.8 gof N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester, removal of thetoluene by distillation was started at an inside temperature of 60 to70° C. and, finally, at an internal pressure of 20 mm Hg and an insidetemperature of 68° C., there was obtained 30.3 g of an oil containingN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester. To this was added29.6 g of tetrahydrofuran, to give 59.9 g of a tetrahydrofuran solutionof N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester. Thistetrahydrofuran solution contained 29.8 g (concentration: 49.7% byweight) of N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester; theyield was 100%, the optical purity was 99.6% and the amount of residualtoluene was 0.2 g (concentration: 0.3% by weight).

EXAMPLE 13

[0127] From 67.2 g of the extract obtained in Reference Example 5 andcontaining 6.78 g (optical purity: 94.5%) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester and 0.11 g ofN-benzyloxycarbonyldehydroalanine methyl ester, removal of the ethylacetate and toluene by distillation was started at 200 mm Hg and aninside temperature of about 40° C., and, finally at 20 mm Hg and aninside temperature of about 68° C., there was obtained 6.92 g of an oilcontaining N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester. To thiswas added promptly 30 ml of toluene, to give 32.81 g of a toluenesolution. This toluene solution contained 6.76 g ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester and 0.11 g ofN-benzyloxy-carbonyldehydroalanine methyl ester; the yield was 100% andthe optical purity was 94.5%.

[0128] A 32.33 g portion of this toluene solution (containing 6.66 g ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester and 0.11 g ofN-benzyloxycarbonyldehydroalanine methyl ester) was cooled to 25° C. andthen 30 ml of hexane was added dropwise over 3 hours with vigorousstirring, followed by further cooling to 5° C. The resulting precipitatecrystals were filtered off, the wet crystals obtained were washed with10 ml of hexane and then dried under reduced pressure (1 to 30 mm Hg, 20to 40° C., 10 hours) to give 5.43 g (yield: 81%) of white crystals. Withthese crystals, the purity of N-benzyloxycarbonyl-S-phenyl-L-cysteinemethyl ester was 99.5% by weight and the optical purity was 98.7%. Thecontamination with N-benzyloxycarbonyldehydroalanine methyl ester wasnot detected.

Reference Example 6

[0129] One gram each of N-benzyloxycarbonyl-S-phenyl-L-cysteine methylester samples differing in optical purity was dissolved in 5 mL oftoluene, and 5 mL of hexane was added at about 25° C. with stirring.After thorough crystallization, the optical purity of the solublefraction was determined. The results thus obtained are shown in Table 2.TABLE 2 Initial optical Optical purity of Purity (%) soluble fraction(%) 90.5 79.3 81.9 79.9 63.8 79.2

[0130] Regardless of the initial optical purity, the optical purityvalues of the N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl estersamples dissolved showed substantially the same value (79 to 80%).

EXAMPLE 14

[0131] From 67.0 g of the extract obtained in Reference Example 5 andcontaining 6.76 g (optical purity: 94.5%) ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester and 0.11 g ofN-benzyloxycarbonyldehydroalanine methyl ester, removal of the ethylacetate and toluene by distillation was started at 200 mm Hg and aninside temperature of about 40° C., and, finally, at 20 mm Hg and aninside temperature of about 68° C., there was obtained 6.95 g of an oilcontaining N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester. To thiswas added promptly 16 ml of methanol, to give 19.71 g of a methanolsolution. This methanol solution contained 6.74 g ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester and 0.11 g ofN-benzyloxy-carbonyldehydroalanine methyl ester; the yield was 100% andthe optical purity was 94.5%.

[0132] A 18.66 g portion of this methanol solution (containing 6.38 g ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester and 0.11 g ofN-benzyloxycarbonyldehydroalanine methyl ester) was cooled to 5° C. over3 hours with vigorous stirring, and the resulting crystals were filteredoff. The wet crystals obtained were washed in sequence with a mixedsolution composed of 10 ml of methanol and 10 ml of pure water and with20 ml of pure water and then dried under reduced pressure to give 4.55 g(yield: 71%) of white crystals. With these crystals, the purity ofN-benzyloxycarbonyl-S-phenyl-L-cysteine methyl ester was 99.0% by weightand the optical purity was 99.7%. Contamination withN-benzyloxycarbonyldehydroalanine methyl ester was not detected.

Reference Example 7

[0133] One gram each of N-benzyloxycarbonyl-S-phenyl-L-cysteine methylester samples differing in optical purity was added to 15 mL of methanoland dissolution was effected at about 25° C. with stirring. Thissolution was cooled and, after thorough crystallization, the opticalpurity of the soluble fraction was determined. The results thus obtainedare shown in Table 3. TABLE 3 Initial optical Optical purity of Purity(%) soluble fraction (%) 90.5 86.3 81.9 85.3

[0134] Regardless of the initial optical purity, the optical purityvalues of the N-benzyloxycarbonyl-S-phenyl-L-cysteine methyl estersamples dissolved showed substantially the same value (85 to 87%).

Industrial Applicability

[0135] The production process of the present invention, which is asmentioned above, is economically advantageous, achieves high productionefficiency on a commercial scale as well, and enables production of theoptically active cysteine derivatives with particularly high opticalpurity and good quality.

1. A process for producing an optically active cysteine derivative represented by the general formula (2):

wherein R¹ represents an amino-protecting group of the urethane or acyl type, R⁰ represents a hydrogen atom or, taken together with the above R¹, an amino-protecting group, R² represents a univalent organic group selected from the group consisting of a substituted or unsubstituted alkyl group containing 1 to 7 carbon atoms, a substituted or unsubstituted aryl group containing 6 to 10 carbon atoms and a substituted or unsubstituted aralkyl group containing 7 to 10 carbon atoms, R³ represents a univalent organic group capable of functioning as an ester-type carboxyl-protecting group by its being included in the structure represented by —COOR³ and * represents the position of an asymmetric carbon atom, which comprises reacting a d-form or l-form optically active amino acid derivative represented by the general formula (1):

 wherein R⁰, R¹, R² and * are as defined above, with an alcohol represented by the general formula (3): R³OH  (3)  wherein R³ is as defined above, and a strong acid and/or a thionyl halide to synthesize a D-form or L-form optically active cysteine derivative represented by said general formula (2), and recovering the above optically active cysteine derivative (2) from the reaction mixture, the procedural series from said reaction to said recovery being carried out under a condition such that the medium contacting the above optically active cysteine derivative (2) is within the range from acidic to weakly basic to thereby recover the above optically active cysteine derivative (2) from the reaction mixture while suppressing the decomposition and racemization thereof.
 2. The process for producing an optically active cysteine derivative according to claim 1, wherein the decrease in optical purity during the procedural series from reaction to recovery is not more than 2%.
 3. The process for producing an optically active cysteine derivative according to claim 1 or 2, wherein the strong acid remaining in the reaction mixture and/or hydrogen halide and sulfur dioxide formed as a byproduct are/is eliminated by neutralization using a base.
 4. The process for producing an optically active cysteine derivative according to claim 1, 2 or 3, wherein the recovery of the optically active cysteine derivative of the general formula (2) from the reaction mixture is effected by extracting, using an extraction solvent, said optically active cysteine derivative from the reaction mixture as such or from the reaction mixture after reduction of the reaction solvent content.
 5. The process for producing an optically active cysteine derivative according to claim 4, wherein addition of an inorganic salt and/or warming treatment is applied to the mixture composed of an organic phase and an aqueous phase as resulting from extraction of the optically active cysteine derivative of the general formula (2) using an extraction solvent, to thereby cause separation of said organic phase and aqueous phase, and to recover said organic phase containing said optically active cysteine derivative.
 6. The process for producing an optically active cysteine derivative according to claim 4 or 5, wherein at least one of aromatic hydrocarbons and acetate esters is used as the extraction solvent.
 7. The process for producing an optically active cysteine derivative according to claim 4, 5 or 6, wherein, in concentrating the organic phase containing the optically active cysteine derivative of the general formula (2), the solvent in coexistence is distilled off at a temperature not lower than the melting temperature of said optically active cysteine derivative or at a temperature not lower than the melting temperature of the concentrate comprising said optically active cysteine derivative as the main component, to thereby recover said optically active cysteine derivative in the form of an oily substance.
 8. The process for producing an optically active cysteine derivative according to claim 7, wherein a solvent is further added to the optically active cysteine derivative of the general formula (2) as obtainable in the form of an oily substance, to give a solution of said optically active cysteine derivative.
 9. The process for producing an optically active cysteine derivative according to claim 4, 5, 6, 7 or 8, wherein, in causing the optically active cysteine derivative of the general formula (2) to precipitate from the organic phase containing said optically active cysteine derivative, adding an aliphatic hydrocarbon as a poor solvent to said organic phase or a concentrate of said organic phase, and recovering the optically active cysteine derivative.
 10. The process for producing an optically active cysteine derivative according to claim 9, wherein the aliphatic hydrocarbon is hexane.
 11. The process for producing an optically active cysteine derivative according to claim 1, 2 or 3, wherein the optically active cysteine derivative of the general formula (2) is recovered from an alcohol, water, or a mixed solvent composed thereof, without performing any extraction procedure.
 12. The process for producing an optically active cysteine derivative according to claim 11, wherein the optically active cysteine derivative of the general formula (2) is recovered from the reaction mixture containing the alcohol of the general formula (3), water, or a mixed solvent composed thereof, either as such or distilled off said alcohol and/or supplemented with water so that the content of water in said reaction mixture may become not lower than 10% by weight relative to the total amount of said alcohol and water.
 13. The process for producing an optically active cysteine derivative according to claim 1, 2 or 3, wherein, when a compound represented by the general formula (4):

 wherein R² and * are as defined above, coexists, as a contaminant, in the optically active amino acid derivative of the general formula (1), R¹ being benzyloxycarbonyl, said compound of the general formula (4) is caused to remain substantially unreacted in the reaction step.
 14. The process for producing an optically active cysteine derivative according to claim 13, wherein the compound of the general formula (4), which remains unreacted, is removed from the reaction mixture.
 15. The process for producing an optically active cysteine derivative according to claim 14, wherein the compound of the general formula (4) is removed by filtration and/or a phase separation procedure comprising extraction and washing.
 16. The process for producing an optically active cysteine derivative according to claim 15, wherein the phase separation procedure comprising extraction and washing is carried out under an acidic condition with addition of an inorganic salt and/or warming treatment, to thereby causing the compound of the general formula (4) to transfer to the aqueous phase.
 17. The process for producing an optically active cysteine derivative according to any of claims 1 to 12, wherein, in general formula (1), R² is a phenyl group.
 18. The process for producing an optically active cysteine derivative according to claim 17, wherein, in general formula (1), R¹ is a urethane-type protecting group.
 19. The process for producing an optically active cysteine derivative according to claim 18, wherein the urethane-type protecting group is an aralkyloxycarbonyl group or a lower alkoxycarbonyl group.
 20. The process for producing an optically active cysteine derivative according to claim 19, wherein said urethane-type protecting group is benzyloxycarbonyl, tert-butoxycarbonyl, methoxycarbonyl or ethoxycarbonyl.
 21. The process for producing an optically active cysteine derivative according to claim 20, wherein the urethane-type protecting group is benzyloxycarbonyl.
 22. The process for producing an optically active cysteine derivative according to any of claims 13 to 16, wherein, in general formula (1), R² is a phenyl group.
 23. The process for producing an optically active cysteine derivative according to claim 17, 18, 19, 20, 21 or 22, wherein, in general formula (2), R³ is a lower alkyl group containing 1 to 4 carbon atoms.
 24. The process for producing an optically active cysteine derivative according to claim 23, wherein the lower alkyl group containing 1 to 4 carbon atoms is methyl.
 25. A method of crystallization of an optically active cysteine derivative represented by the general formula (2):

wherein R⁰ represents a hydrogen atom, R¹ represents a urethane-type amino-protecting group, R² represents a phenyl group, R³ represents a lower alkyl group containing 1 to 4 carbon atoms and * represents the position of an asymmetric carbon atom, wherein the cysteine derivative of the general formula (2) is caused to precipitate as a crystal from a solution containing the cysteine derivative of the general formula (2) in an organic solvent miscible with an aliphatic hydrocarbon under acidic to weakly basic conditions by using an aliphatic hydrocarbon as a poor solvent, if necessary with cooling and/or concentration, while thereby effecting removal, from the cysteine derivative of the general formula (2), of a contaminant impurity existing therein.
 26. The method of crystallization according to claim 25, wherein at least one of aromatic hydrocarbons, acetate esters and ethers is used as the aliphatic hydrocarbon-miscible organic solvent.
 27. The method of crystallization according to claim 26, wherein toluene is used as the aromatic hydrocarbon, ethyl acetate as the acetate ester, or methyl tert-butyl ether as the ether.
 28. The method of crystallization according to claim 25, 26 or 27, wherein the poor solvent aliphatic hydrocarbon is hexane.
 29. The method of crystallization according to claim 25, 26, 27 or 28, wherein the poor solvent aliphatic hydrocarbon is used in an amount of not less than 10% by weight relative to the total weight of the crystallization mixture.
 30. The method of crystallization according to claim 25, 26, 27, 28 or 29, wherein the lower alkyl group containing 1 to 4 carbon atoms is methyl.
 31. The method of crystallization according to claim 25, 26, 27, 28, 29 or 30, wherein the urethane-type protective group is an aralkyloxycarbonyl group or a lower alkoxycarbonyl group.
 32. The method of crystallization according to claim 31, wherein the urethane-type protective group is benzyloxycarbonyl, tert-butoxycarbonyl, methoxycarbonyl or ethoxycarbonyl.
 33. The method of crystallization according to claim 32, wherein the urethane-type protective group is benzyloxycarbonyl.
 34. The method of-crystallization according to claim 25, 26, 27, 28, 29, 30, 31, 32 or 33, wherein at least one of the optical isomer of the cysteine derivative (2) and a dehydroalanine derivative represented by the general formula (5):

 wherein R⁰, R¹ and R³ are the same as the definition of the general formula (2), is the impurity existing in said cysteine derivative (2).
 35. The method of crystallization according to claim 34, wherein the desired optically active cysteine derivative (2), which exists in excess, is caused to preferentially precipitate out by adjusting a crystallization condition so that the relative equilibrium solubility (ratio between amounts dissolved at equilibrium) in the impurity optical isomer-containing crystallization mixture between the desired optically active cysteine derivative (2) and the optical isomer thereof may be lower than the optical purity of the cysteine derivative prior to crystallization.
 36. The method of crystallization according to claim 35, wherein the existence percentage of the desired optical isomer in the crystals as obtainable is not less than 98%.
 37. A method of crystallization of an optically active cysteine derivative represented by the general formula (2):

wherein R⁰ represents a hydrogen atom, R¹ represents a urethane-type amino-protecting group, R² represents a phenyl group, R³ represents a lower alkyl group containing 1 to 4 carbon atoms and * represents the position of an asymmetric carbon atom, wherein the optically active cysteine derivative of the general formula (2) is caused to precipitate as a crystal from a solution containing the optically active cysteine derivative of the general formula (2) in a monohydric alcohol containing 1 to 4 carbon atoms under acidic to weakly basic conditions, if necessary with cooling and/or concentration, while thereby effecting removal, from the optical active cysteine derivative of the general formula (2), of a contaminant impurity existing therein.
 38. The method of crystallization according to claim 37, wherein the lower alkyl group containing 1 to 4 carbon atoms is methyl and the monohydric alcohol containing 1 to 4 carbon atoms is methanol.
 39. The method of crystallization according to claim 38, wherein the content of water in the methanol solution containing the optically active cysteine derivative of the general formula (2) is less than 10% by weight relative to the total amount of methanol and water.
 40. The method of crystallization according to claim 37, 38 or 39, wherein the urethane-type protective group is an aralkyloxycarbonyl group or a lower alkoxycarbonyl group.
 41. The method of crystallization according to claim 40, wherein the urethane-type protective group is benzyloxycarbonyl, tert-butoxycarbonyl, methoxycarbonyl or ethoxycarbonyl.
 42. The method of crystallization according to claim 41, wherein the urethane-type protective group is benzyloxycarbonyl.
 43. The method of crystallization according to claim 37, 38, 39, 40, 41 or 42, wherein at least one of the optical isomer of the optical active cysteine derivative of the general formula (2) and a dehydroalanine derivative represented by the general formula (5):

 wherein R⁰, R¹ and R³are the same as the definition of the general formula (2), is the impurity existing in said optical active cysteine derivative (2).
 44. The method of crystallization according to claim 43, wherein the desired optically active cysteine derivative (2), which exists in excess, is caused to preferentially precipitate out by adjusting a crystallization condition so that the relative equilibrium solubility (ratio between amounts dissolved at equilibrium) in the impurity optical isomer-containing crystallization mixture between the desired optically active cysteine derivative (2) and the optical isomer thereof may be lower than the optical purity of said cysteine derivative prior to crystallization.
 45. The method of crystallization according to claim 44, wherein the existence percentage of the desired optical isomer in the crystals as obtainable is not less than 98%. 